The invention is situated in the field of piece goods conveyance and processing and it relates to a method and a device in accordance with the generic terms of the corresponding independent claims. Method and the device according to the invention serve for forming groups of flat articles, in particular of printed products, wherein the flat articles are supplied in a plurality of substantially continuous streams, wherein every group being conveyed away usually contains one article from every supply stream and wherein the flat articles in each group are arranged in a stack-like manner.
Group formation as mentioned is a frequently used step during further processing of products being produced by printing machines. For example, signatures are collated to form such groups (each group representing one book) or other printed product parts being produced in separate printing processes, or finished printed products to be combined into shipping units or enclosures to be inserted into a main product. Usually, products differing from one another are collated in such groups and there is one supply stream for each product type.
In accordance with prior art, stack-shaped groups of printed products are formed along a collating stretch. The groups or stacks under formation are conveyed along the collating stretch one behind the other lying on a substantially horizontal support or being accommodated in conveying compartments, in which the flat articles are leant against an inclined wall. In this manner, the groups or stacks being produced are transported past feed points, wherein at every feed point a supply stream merges into the collating stretch and usually at every feed point a printed product is added to each group being transported past.
It is obvious, that conveying the stacks or groups under creation one behind the other on a substantially horizontal conveying surface can be implemented with simple devices, for example, with a conveyor belt comprising simple compartments following one another in the conveying direction. It is also obvious, however, that this type of collating stretch becomes relatively long, the length of the stretch being dependent on the largest formats of the printed products to be handled and on the number of feed points. Furthermore, supply of the printed products to stacks being transported one behind the other is easy to implement, for example, directly from imbricated streams, in which the printed products are supplied with their leading edges positioned on the upper stream surface and for each supply step the article forming the front end of the stream is pushed or falls downwards on to the stack being transported past. If, however, with installations of the named kind high capacities are to be achieved, high conveying speeds along the collating stretch become necessary, which is disadvantageous with respect to the energy required. Furthermore, strong air currents are created and necessitate in particular for light articles of relatively large formats, holding of the stacks during conveyance, and therefore releasing them for every further addition of a product.
The second, above mentioned method, in which the collated printed products are leaning in conveying compartments (lying stacks) constitutes an attempt to avoid the mentioned difficulties. The expanse in conveying direction of these conveying compartments is limited not by the largest product format to be expected, but rather by the largest stack height (sum total of the thicknesses of the printed products stacked in each group) to be expected, which for most applications is significantly smaller than the two-dimensional product expanse (format). However, to be added to the groups, the printed products have to be introduced into the conveying compartments, i.e. between the stacks. For this purpose, the products are usually conveyed, each one being held gripped at an upper edge by a gripper and are introduced into the conveying compartments from above, i.e. with the unguided, lower edge leading. If, for shortening the collating stretch, the conveying compartments are as narrow as possible, space for supplying the products is small and the supply action needs to be correspondingly precise. This on the one hand calls for a low relative speed between the conveying compartment and the article to be added and therefore for a relatively long supply stretch and for a limited absolute speed of the printed product to be supplied, such that the unguided edge being the leading edge during the supply operation is not displaced in an uncontrolled manner by the air current. From the above follows, that the second one of the above described methods requires significantly more expensive mechanical means and therefore, can only solve the above mentioned problems partially.
A combination of simplicity with respect to the needed device and a short, i.e., space saving collating stretch (tight stack arrangement and close succession of feed points) is conceivable, if the stacks overlap one another, i.e., if they are trans-ported in the manner of an imbricated stream of stacks, i.e. in a stream which lies on a substantially horizontal conveying surface and in which every scale comprises a plurality of articles lying one on top of the other (stacked). Solutions in this direction are described in the publications DE-3145491 (or U.S. Pat. No. 4,471,953) and DE-19643395 (or U.S. Pat. No. 5,727,781).
In accordance with DE-3145491, the stacks overlapping one another are transported along the collating stretch with the help of stack grippers, which act on the trailing stack sides, wherein the articles of a succeeding stack may cover a downstream stack gripper. For adding a further article to a stack being transported in the named manner, the corresponding stack gripper is opened towards the top and thereby the articles of the succeeding stack, which are covering the gripper are lifted. The articles to be deposited on the stacks are supplied in a supply direction and being individually held gripped at upper, leading edges. Therein the supply direction is parallel to the conveying direction of the collating stretch and the supply speed is lower than the speed of the imbricated stacks along the collating stretch. The article supply is controlled in such a manner, that the unguided trailing edge of an article to be added to a stack is positioned on a guide surface or on an already stacked article and as a result of the speed difference is pushed into the stack gripper and therewith underneath the lifted articles of the succeeding stack. As soon as the trailing edge of the article makes contact with the gripper body, i.e. is aligned with the trailing edges of articles already stacked in the stack gripper, the supply gripper is opened and the stack gripper is closed.
The above description demonstrates, that in accordance with DE-3145491 a relatively simple stack conveyance with small stack spacings becomes possible. However, it is not possible to arrange the feed points very closely following one another in the direction of stack conveyance, the device is expensive and the feed points have to be equipped in dependence of article format.
Publication DE-19643395 also describes a collating method based on the idea of an imbricated stack stream. The rectangular, or if so applicable, square printed products to be supplied at a feed point are not really positioned on a stack being transported past the feed point, but they are rather laid, in the form of an imbricated stream, on a group stream comprising one or several imbricated streams. Therein, in all superimposed, imbricated streams, scale spacings are the same and leading zones of the printed products belonging to the same group are aligned to one another but are separated from one another by trailing zones of printed products of preceding groups. At a head end of such an imbricated group stream, where leading zones of printed products belonging to a front-most group are positioned on top of one another (there is no preceding group, the articles of which are lying in between), these leading zones are gripped and accelerated, as a result of which the front-most group is pulled out of the stream as a stack and is separated from the stream.
For an unproblematic group separation, DE-19643395 proposes to laterally guide the stacks being pre-formed in the group stream, in order to align the printed products which belong to a specific group to be formed, but which belong to different ones of the superimposed imbricated streams. For making such guidance possible, in the group stream, the printed products are arranged diagonally, i.e., with a leading corner. The longitudinal edges of a group stream of such diagonally arranged, rectangular or square printed products are not straight, but serrated, so that the individual scales or groups respectively can be guided in the region of their edges being arranged on both stream sides by guide means acting at the serrations (e.g., guide pins being conveyed together with the stream).
The printed products to be supplied to the group stream with a diagonal imbricated arrangement are supplied in supply streams, in which the product edges are aligned perpendicular and parallel to the conveying direction and which meet the stack stream at a slant where every printed product is deviated into the direction of the diagonal conveyance for being positioned on the group stream. For such deviation, forces have to be applied to the printed products, which forces are absorbed by the guide pins.
The position of the guide pins has to be adapted to the largest expected format of printed products to be stacked and to the scale spacing. All products with a smaller format are aligned to one or the other of the guided corners depending on the supply direction and they are guided on one side only. Very small products if so applicable remain completely unguided. Supply is only possible from above and it is a condition that in the supply streams the leading product edges are located on the upper stream side.
The object of the invention is to create a method and a device, which serve for forming groups of flat articles supplied in supply streams and which, like the system according to DE-19643395, are based on a group stream comprising a plurality of superimposed, imbricated streams. The method and the device according to the invention, however, shall bring improvements over the prior art not only with respect to independence of the format, to flexibility and to simplicity with respect to the devices, but also with regard to the shortness of the collating stretch. In particular, the alignment of the articles in the groups to be created shall be much less fixedly predefined by the method or by the device, but shall to a great extent be adaptable to the characteristics of the articles and to the stability of the stacks.